The UN's nuclear watchdog said Wednesday it could not guarantee the peaceful nature of Iran's nuclear program, saying there had been "no progress" in resolving questions over the past presence of nuclear material at alleged "undeclared sites".

In a report seen by AFP, the International Atomic Energy Agency (IAEA) said it was "not in a position to provide assurance that Iran's nuclear program is exclusively peaceful."

It said IAEA Director General Rafael Grossi was "increasingly concerned that Iran has not engaged with the Agency on the outstanding safeguards issues during this reporting period and, therefore, that there has been no progress towards resolving them."

The IAEA has been pressing Iran for answers on the presence of nuclear material at three alleged "undeclared sites".

In a separate report also issued on Wednesday, the IAEA said that Iran was continuing to enrich uranium well over the limits laid down in the 2015 deal, with its stockpile now over 19 times the limit set out in the accord.

The report claimed that Iran's stockpile as of August 21 stood at an estimated 3,940 kilograms, up 131.6 kilograms from the last quarterly report, "over 19 times the limit set out in the accord."

Iran urges IAEA to drop "baseless accusations"

It is noteworthy that in late August, Iran's top diplomat Foreign Minister Hossein Amir-Abdollahian demanded that the IAEA drop the issue of what the Western parties claim to be "undeclared sites", as momentum builds to revive a 2015 nuclear deal.

In June, the IAEA's board of governors adopted a resolution censuring Iran for failing to adequately explain the previous discovery of traces of enriched uranium at three sites allegedly "not declared by Tehran" as having hosted nuclear activities.

"We are very serious about safeguard issues, and do not want to allow some of the IAEA's baseless accusations to remain," IRNA quoted Amir-Abdollahian as saying.

Read more: IAEA demands excessive: Iran Atomic Energy Organization